diff --git a/core/mrtrix.cpp b/core/mrtrix.cpp
index 4925121620..328d09b7bf 100644
--- a/core/mrtrix.cpp
+++ b/core/mrtrix.cpp
@@ -233,8 +233,8 @@ size_t char_is_dash(const char *arg) {
   return 0;
 }
 
-size_t is_dash(const std::string &arg) {
-  size_t nbytes = char_is_dash(arg.c_str());
+bool is_dash(const std::string &arg) {
+  const size_t nbytes = char_is_dash(arg.c_str());
   return nbytes != 0 && nbytes == arg.size();
 }
 
diff --git a/core/mrtrix.h b/core/mrtrix.h
index c8e0579e33..efe5e19ba4 100644
--- a/core/mrtrix.h
+++ b/core/mrtrix.h
@@ -105,7 +105,7 @@ bool match(const std::string &pattern, const std::string &text, bool ignore_case
 size_t char_is_dash(const char *arg);
 
 //! match whole string to a dash or any Unicode character that looks like one
-size_t is_dash(const std::string &arg);
+bool is_dash(const std::string &arg);
 
 //! match current character to a dash or any Unicode character that looks like one
 /*! \note If a match is found, this also advances the \a arg pointer to the next